In cold districts of Europe and America, and in areas of heavy snowfall of Hokuriku, Tohoku and Hokkaido districts of Japan, slip accidents of automobiles occur by the freezing of roads in winter, so that rock salt is scattered for preventing the freezing of roads by virtue of the action of its freezing-point depression. This is useful for preventing the freezing of roads. However, rock salt contains a large amount of sodium chloride. Accordingly, when passing automobiles send rock salt flying and it strikes their own automobiles, sodium chloride remains on outer panels of the automobiles to develop rust on steel panels thereof, which causes hole corrosion and structural corrosion in the end.
As a measure for solving this problem, vehicles, particularly outer panels and parts of automobiles, are coated with wax antirusts for preventing rust. These antirusts are ones mainly composed of microcrystalline wax or petrolatum oxide, with which various antirust additives and pigments are mixed. They, of course, cover surfaces of the steel panels, and penetrate joints between the steel panels to fill them, thereby protecting the steel panels from rust due to moisture entering such sites. Thus, they have a high antirust effect. However, for obtaining their penetrating performance, it is generally required that coatings formed are flexible, and the effect for preventing the coatings from chipping due to pebbles sent flying is insufficient. For this reason, methods have been employed in which phthalic acid alkyd resin coatings are first applied to form hard coatings, and then, wax antirusts are applied thereto. However, the wax antirusts are relatively weak against steam washing which is conducted for removing snow or mud adhered to bodies, so that they have the disadvantage that the repetition of steam washing results in a gradual reduction in antirust ability.
As an anti-chipping measure, outer panels of body floors and wheel houses are coated with vinyl chloride plastisols obtained by plasticizing polyvinyl chloride resins with appropriate plasitcizers and mixing fillers therewith, as under body coatings, and side sills, fenders, aprons and lower portions of doors of bodies are coated with one-part type polyurethane resin coatings or polyester resin coatings, as anti-chipping coatings. These coatings have both the flexibility and toughness, so that they have a considerable anti-chipping effect. However, the coatings themselves have no substantial antirust effect. Accordingly, they are effective for chipping of pebbles and grit, but insufficient as a measure for chipping of rock salt. Further, they have little penetration, so that it has been necessary to coat joints between the steel panels with the conventional wax antirusts, separately.
Japanese Unexamined Patent Publication No. Hei 6-299026 shows an approach for enhancing antirust ability of under coatings such as vinyl chloride plastisols. In this improvement, however, the antirust performance is basically added while maintaining the anti-chipping performance and shock-resistant performance, and the long-term antirust performance by the penetration into joints between the steel panels is not given.
Further, various antirust steel panels improved in antirust performance by placing a special resin between two steel panels are developed in various iron manufacturing companies. However, these steel panels have the disadvantage of being high in cost, compared with ordinary steel panels.
Summarizing these, in the present circumstances, wax antirusts and their improved type antirusts satisfying the anti-chipping performance and resistance to steam washing have not been found yet, whereas highly reliable antirust ability is not given to the under body coatings of the vinyl chloride plastisol family.